Optical fibres are increasingly being used to replace wired communications such as twisted pair and coaxial cable, as well as to enable new applications which were previously impractical such as conveying millimeter wave signals over any significant distance. Optical fibre based systems have already found widespread application in digital signal transmission. More recently, the transmission of analog signals over optical fibres has grown in importance.
In the field of modern telecommunications, the emergence of fibre optics as the transmission system of choice can be attributed to two factors: the single mode fibre in these systems has virtually unlimited analog and digital signal bandwidth, and a life measured in decades. There are primarily three wavelengths that are used in fibre optic links. All are in the near-infrared portion of the electromagnetic spectrum. By far the dominant wavelengths in use at present are located in bands around 1310 and 1550 nm. The choice of these particular wavelength bands is based on the availability of certain desirable fibre properties, such as low attenuation (1550 nm) and zero wavelength dispersion (1310 nm).
A large proportion of the worldwide installed fibre base consists of conventional fibre that is optimised for near-zero dispersion in the second transmission window of 1310 nm. Current transmission systems operate in the third transmission window of 1550 nm, however, to take advantage of low-loss transmission and the availability of optical amplifiers.
If an optical signal is to travel over long distances, it is necessary to use amplifiers to strengthen the signal at intervals. These amplifiers are needed because, over long distances, the signal tends to fade, or attenuate and in the case of digital transmission each optical pulse tends to spread out from the more compact form in which it was transmitted. Before the development of optical amplifiers, the only way to boost an optical signal was to regenerate it electronically. That is, convert the optical signal to an electrical signal, amplify it, convert it back to an optical signal and then retransmit it. Optical amplifiers, although still generally requiring both electrical power to drive the pump lasers and electronic systems to provide stability and monitoring functions, are much less costly than electrical amplifiers because they do not have to regenerate the individual optical signals.
Optical amplifiers may be classified into two categories, namely laser-diode amplifiers and doped-fiber amplifiers. The latter are particularly attractive because of their ease of manufacture and simplicity of coupling into the fiber link. Erbium doped-fiber amplifiers (EDFAs), for example, are pieces of fibre that are doped with erbium, an element that can boost the power of an optical wavelength. In fact, it can simultaneously amplify all the wavelengths on a given fibre, and it may do so passively (i.e. without electrical power or electronic systems).
Most of the long-haul telecommunications traffic today is carried on synchronous optical networks. There is escalating pressure on global telecommunications networks to provide higher transmission capacity, achievable by increasing data transmission rates. Although this sounds attractive from a systems point of view, successful deployment of such systems depends on resolving a number of critical issues, foremost among them chromatic dispersion, which can introduce errors in the data stream if left unchecked.
Chromatic dispersion in lightwave systems is caused by a variation in the group velocity in a fibre with changes in optical frequency. A data pulse generally contains a spectrum of wavelengths introduced by modulation. Dispersion is a phenomenon that results from the fact that light of different wavelengths travels with different velocities through optical fibre. Since the speeds are different, wavelengths having higher velocity tend to move toward the front of the pulse, while wavelengths having a slower velocity move toward the rear. This causes the entire pulse to spread out, decreasing the clarity of the signal, and causing further problems (i.e. introduction of errors in the data) if the pulse is actually intermingled with other pulses.
Chromatic dispersion thus places a practical limit on the distance a signal can be transmitted before some form of compensation becomes necessary. At rates of 10 Gbit/s this distance is about 60 km, and at higher data rates the distance shortens. A number of solutions have been proposed to remedy this problem, including dispersion compensating fibre (DCF), chirped fibre gratings, mid-span spectral inversion, multilevel coding, and others. However, only dispersion-compensating fibre and chirped fibre gratings have been considered seriously as potential candidates for practical deployment.
When light of 1550 nm wavelength is used in conventional 1310 nm single mode transmission fibre, high levels of chromatic dispersion (approximately 17 ps/nm/km) are introduced. If erbium-doped fiber amplifiers (inherently analog devices) are employed, the chromatic dispersion accumulates, becoming the primary limiting factor to capacity. The simplest way to compensate for the total accumulated dispersion in a fibre link is to concatenate standard fibre with a fibre introducing high levels of dispersion of the opposite sign. This dispersion compensating fibre (DCF) compensates for the cumulative positive dispersion with its negative dispersion characteristic. DCF has a large negative chromatic dispersion at a wavelength of 1550 nm and is typically five to ten times as dispersive as conventional transmission fibre. A typical DCF, for example, has a dispersion value −100 ps/nm/km. For example, to ‘completely’ compensate for the dispersion introduced by an 80 km span, a system requires about 13 km of DCF. It should be noted here that ‘complete’ dispersion compensation doesn't necessarily mean zero net dispersion as many systems exploit the interplay between fiber dispersion and transmitter chirp to allow for additional reach. Complete compensation simply means the optimum degree of net dispersion to suit the particular transmitter incarnation chosen.
DCF also suffers from higher attenuation than conventional fibre. Over the long lengths required, the fibre introduces significant losses, typically 5 to 8 dB over an 80 km span. Therefore, additional optical amplifiers must be provided to compensate for this loss. Key components, then, in any long haul optical transport communications system are the optical amplifiers (e.g. EDFAs) and dispersion compensating modules (DCMs), of which DCF is only one realisation.
If carefully constructed, an optical fibre communication systems can provide low loss and very high bandwidth information carrying capacity. In practice the bandwidth of optical fibre may be utilised by transmitting many distinct channels simultaneously using different carrier wavelengths. The associated technology is called wavelength division multiplexing (WDM). The advent of wavelength division multiplexing as the technology of choice for upgrading the capacity of fibre-optic networks has created a demand for devices and components capable of pushing that capacity to its limit. For maximum upgrade flexibility, the fibre selected for a high capacity network should minimize optical non-linear effects, such as four-wave mixing and keep dispersion (pulse spreading) to a minimum.
The reach and capacity of unregenerated multichannel optical wavelength division multiplexed (WDM) transport systems are limited by the combined effects of dispersion, noise and fibre non-linearities. Current WDM products tend to have a relatively low channel count, which means that the channels can be spaced relatively widely (typically 100-200 GHz) within the gain window of erbium-doped fibre amplifiers (EDFAs). However, as the demand for bandwidth continues to grow, there is a need to provide more and more capacity over a single fibre, and one relatively simple way to achieve this is to reduce the channel spacing so that more channels can be packed within the EDFA gain window. However, when this is done, the crosstalk between channels due to interactions mediated by the fibre non-linearity increases, and eventually imposes a limitation on the maximum reach that can be achieved with a given channel spacing.
The non-linear effect responsible for this limit is four-wave mixing (FWM). This process is due to the fundamental non-linearity of the glass fibre transmission medium, and occurs when three photons (from two or three different wavelength channels) mix to produce a fourth photon at another wavelength. If the wavelength of this fourth photon coincides with that of another communication channel, it cannot be distinguished from the information in that channel and so constitutes unwanted crosstalk.
Therefore, the most significant limiting process in ultra dense WDM optical transmission systems is the high degree of crosstalk between channels due to FWM, and the reason for this effect dominating system performance is the small channel spacing used. This effect rapidly increases as one attempts to further reduce the channel spacing (and increase capacity and spectral efficiency). The FWM process, then, imposes a limitation on the maximum reach, capacity and spectral efficiency that can be achieved in a very densely packed WDM system.
Currently, the problems associated with four-wave mixing are controlled through system design. Specifically, this includes the use of relatively low channel counts, a relatively wide channel spacing and fibres with a reasonable degree of dispersion. The useable wavelength range for long haul high capacity transport is confined to the EDFA gain band. As more and more capacity is demanded of systems, it is extremely likely that channel counts will increase and channel spacings decrease (to achieve ever-increasing capacity on a fibre), rendering the aforementioned measures ineffective. Therefore, the system impact of four-wave mixing will likewise increase, and eventually will limit the maximum capacity and/or reach of dense WDM transport systems.